1. Field of the Invention
The present invention relates to a dot line printer including a printing unit making a reciprocating motion and a shuttle unit making a motion in a direction opposite thereto.
2. Description of the Related Art
A dot line printer apparatus performs printing while a printing unit makes a rectilinear reciprocating motion. Then, when completing one-line printing, the consecutive printing is carried out by feeding a sheet in a direction orthogonal thereto. This printing unit has a plurality of dot elements and is therefore large in weight. Hence, the apparatus oscillates due to the reciprocating motions of the printing unit.
For preventing this phenomenon, in the apparatus, provided is a shuttle unit making a motion in a direction opposite to that of the printing unit. The motion of this shuttle unit takes a balance with the motion of the printing unit. The oscillations of the apparatus can be thereby prevented. In the dot line printer equipped with the shuttle unit making the motion in this opposite direction, the reciprocating motion is required to increase in terms of velocity.
FIG. 8 is a perspective view showing the prior art. FIG. 9 is a sectional view of the prior art.
As illustrated in FIGS. 8 and 9, a balance shuttle unit 9 is provided in a face-to-face relationship with a printing shuttle unit 8.
The printing shuttle unit 8 includes a printing unit 80 having a plurality of dot printing elements. This printing unit 80 is mounted on a shuttle frame 81 moving along a shuttle shaft 85. The printing unit 80 is constructed such that a plurality of wire dot element strings each having a plurality of vertically arranged wire dot elements are arrayed at a predetermined interval.
Coils 82 are provided under this shuttle frame 81 through a yoke 86. A magnet 84 is attached to a base frame (yoke) 83 in a face-to-face relationship with this coil 82. Note that the numeral 87 designates a guide roller for the reciprocating motion, and 88 represents a balance weight.
On the other hand, the balance shuttle unit 9 has a balance weight unit 90 for taking a balance in terms of weight with the printing unit 80. This balance weight unit 90 is mounted on a shuttle frame 91 moving along a shuttle shaft 95. Coils 92 are provided under this shuttle frame 91 through a yoke 96. A magnet 94 is attached to a base frame (yoke) 93 in the face-to-face relationship with the coils 92. Note that the numeral 97 denotes a guide roller for the reciprocating motion.
This printing mechanism is based on the principle that the printing unit 80 is reciprocated by a linear motor. That is, the shuttle unit 81 to which the coils 82 are fixed makes the reciprocating motion in the direction right-angled to a feeding direction of a sheet 71. With this motion, the wire dot printing elements of the printing unit 80 are driven toward a platen 70, thus effecting the printing on the sheet 71.
According to a kinetic principle of the linear motor, an electric current flows across the coils 82 in a magnetic field generated by a permanent magnet 84 disposed under the coils 82. With this operation, the shuttle unit 81 fitted with the coils 82 makes a motion according to the Fleming's left-hand rule.
On the other hand, the balance shuttle unit 9 makes a motion in a direction opposite to that of the printing shuttle unit 8 on the basis of the same principle as that of the printing shuttle unit 8. This offsets an inertial moment of the printing shuttle unit 8, thereby preventing the oscillations.
An improvement of this printing speed entails a speed-up of the reciprocating motion of the printing unit. For this purpose, there are considered a method of reducing a load on the linear motor by decreasing weights of members for making the reciprocating motion and a method of increasing an output of the linear motor.
The former method requires reductions in weight of the coils 82, 92, the shuttle frames 81, 91 and the coil bases 86, 96. Reducing the weights of the coils 82, 92 leads to down-sizing of the coils 82, 92. However, this brings about a drop of output of the linear motor, and therefore the coils can not be reduced in weight. Further, the shuttle frame 81 is hard to extremely decrease in weight because of bearing a printing reaction and, besides, actualizing the stable reciprocating motion.
Moreover, according to the prior art, the coil bases 86, 96 serves as the yokes. For this reason, the coil base is required to have a minimum plate thickness enough not to saturate a magnetic flux of the magnet. Accordingly, this conduces to a problem in which the weight of the coil base is hard to decrease.
On the other hand, increasing the output of the linear motor involves enlarging the coils or the magnet. Consequently, there arises a problem in which the apparatus augment in size.
Besides, according to the prior art, the printing shuttle unit 8 and the balance shuttle unit 9 are disposed facing each other through the sheet, resulting in such a problem that the size of the apparatus increases.